I had short stack Powerflow installed about two years ago at a Palo Alto shop. I noticed a big difference. The engine would run into the "red arc" on rpm so we repitched the prop... straight and level it will go into the yellow arc on speed!
Definitely faster than stock; sounds more like my friends' Mooney. I haven't been flying much lately and don't have numbers but just flying slow around the S.F. Peninsula the throttle stays closer to closed than it used to and seems like fuel burn is less.

I would do it again if I had about 7500 burning a hole in my pocket. You end up with a new exhaust instead of worrying about your old small bore exhaust... plus it looks great. Thanks Powerflow.